Silicon carbide schottky diode

ABSTRACT

A SiC Schottky diode which includes a Schottky barrier formed on a silicon face 4H-SiC body.

RELATED APPLICATIONS

This application is a continuation of and claims the benefit ofco-pending, commonly-owned U.S. patent application Ser. No. 11/581,536,with Attorney Docket No. VISH-IR257, filed on Oct. 16, 2006, by GiovanniRichieri, and titled “Silicon carbide schottky diode,” which claims thebenefit of and priority to the provisional patent application, Ser. No.60/728,728, with Attorney Docket No. VISH-IR257.Pro, filed on Oct. 20,2005, by Giovanni Richieri, and titled “Silicon carbide schottky diode,”each of which are hereby incorporated by reference in their entirety.

FIELD OF INVENTION

The present invention relates to Schottky diodes and in particular toSiC Schottky diodes.

BACKGROUND ART

Although the main intrinsic parameters in Silicon Carbide material havenot been exhaustively studied, several experimental and theoreticalstudies have been performed in recent years in order to better describethe current transport in ohmic and rectifying contact on SiC.

It has been known that in Schottky diodes the metal semiconductorinterface (MST) between the Schottky barrier metal and the semiconductorplays a crucial role in the electrical performance of electronicdevices. Many factors can worsen the performance of the MSI in aSchottky diode. For example, the quality of the semiconductor surfaceprior to the deposition of the Schottky barrier metal can cause thedevice to exhibit characteristics that are different from the idealcharacteristics.

Current-voltage (I-V) and capacitance-voltage (C-V) characterizationsare useful methods for determining the Schottky barrier height fromwhich the MSI uniformity can be obtained. It has been known that thebarrier height determined from C-V measurements often exceeds the valueextracted from forward I-V measurements. The discrepancy is also seen tobe greater for Schottky contacts with high ideality factor, i.e. poorSchottky MSI quality. Sullivan et al., J. Appl. Physics, 70, No. 12,(1991), 7403-7424, have explained that such a discrepancy is due tobarrier height inhomogeneities, and have demonstrated that if thebarrier height is assumed to follow some statistical distribution, thebarrier height value extracted from C-V measurements is the arithmeticmean of the barrier height distribution (the capacitance of the Schottkycontact comes from the modulation of the total space-charge as afunction of bias). On the other hand, the value from I-Vcharacterization is well below the value extracted from capacitanceanalysis, and is dominated by the current that flows through regions oflower barrier height. Also, the inhomogeneity of the barrier height isbelieved to be a primary reason for the observation of large idealityfactors (n>1.1). For intimate contact and uniform MSI, the barrierheight values extracted from C-V and I-V characterization are expectedto be in close correlation, and ideality factors very close to unity.

SUMMARY OF THE INVENTION

A semiconductor device according to the present invention is a SiCSchottky diode which includes a SiC substrate of one conductivity, asilicon face SiC epitaxial body of the one conductivity formed on afirst surface of a the SiC substrate, a Schottky metal barrier formed onthe silicon face of the SiC epitaxial body, a back power electrode on asecond surface of the SiC substrate opposite said first surface of theSiC substrate.

In one embodiment of the present invention the Schottky metal barrier isformed of Titanium, and the silicon face epitaxial SiC is 4H—SiC. It hasbeen found that this combination results in a very uniform MSI, whichallows for accurate electrical characterization of the device.

Other features and advantages of the present invention will becomeapparent from the following description of the invention which refers tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of a portion of a SiC semiconductordevice according to the present invention.

FIG. 2 graphically illustrates the I-V characteristics of a deviceaccording to the present invention as a function of temperature.

FIG. 3 illustrates a Richardson's plot of In(Js/T**2) versus 1/kT,experimental data and linear fit.

DETAILED DESCRIPTION OF THE FIGURES

A power semiconductor device according to the preferred embodiment ofthe present invention is a discrete Schottky diode that includes SiCsubstrate 10 (preferably 4H—SiC bulk SiC) of N-type conductivity,silicon face 4H—SiC epitaxial body 12 of N-type conductivity formed onone surface of substrate 10, Schottky metal body (Schottky barrier) 14formed over and in Schottky contact with epitaxial body 12, and a backpower electrode 16 formed on another opposite surface of substrate 10. Aregion of P-type conductivity 18 serving as a guard ring is diffusedinto epitaxial body 12, is in contact with Schottky body 14, and definesthe other boundary of the active region (i.e. region defined by theSchottky contact between

Schottky body 14 and epitaxial body 12) of the device. A deviceaccording to the preferred embodiment further includes a fieldinsulation body 20 disposed on epitaxial body 12 and surrounding theactive region of the device. Note that field insulation body 20 includessidewalls 22 that taper toward the active region of the device, and thatSchottky body 14 extends over sidewalls 22 and a portion of the topsurface of field insulation body 20. A device according to the preferredembodiment further includes a metallic front power electrode 24 which isdisposed over at least Schottky body 14.

In the preferred embodiment, Schottky body 14 is composed of titanium(which has been observed to make a uniform MSI with silicon face4H—SiC), front power electrode 24 is composed of aluminum, back powerelectrode 16 is composed of a trimetal stack of titanium (in ohmiccontact with substrate 10), nickel (formed on the titanium layer), andsilver (formed on the nickel layer). Field insulation body 20 ispreferably composed of silicon dioxide.

The behavior of the electrical parameters of a Schottky barrier diode(SBD) fabricated on the Si face of a 4H—SiC epitaxial layer according tothe present invention was studied. The study revealed that the devicesaccording to the present invention exhibit an electrical behavior inaccordance to thermoionic emission and a good MSI uniformity confirmedby C-V measurements. I-V measurements (I-V) were also performed in alarge temperature range which led to an evaluation of the Richardsonconstant.

Details of the Study

Schottky barrier diodes were fabricated on 3″4H—SiC wafers, productiongrade, produced by Cree Inc. Epitaxial body 12 and substrate 10 ofwafers were n-doped (Nd=10¹⁶ cm⁻³, thickness 7 μm and Nd=10¹⁸ cm⁻³,thickness 380 μm, respectively). The junction extermination extension onfabricated devices was formed by a guard ring 18 obtained by P⁺ typeimplantation and a mesa structure with an additional ring of siliconoxide 20. Schottky barrier 14 was made by thermal evaporation ofTitanium and a successive Aluminum layer 24. Ohmic contact 16 formationwas made on the back-side of the wafer by a triple evaporation ofTitanium, Nickel and Silver.

Fabricated devices were then packaged in a standard TO-220 commercialpackage.

All step processes were optimized in previous work, leading to obtain apercentage of working diodes with a reverse current<50 μA@600 V ofreverse voltage up to 85%.

Electrical measurements were performed by an SMU237 Keithley SourceMeasure Unit and an SMU238 Keithley Source Measure Unit. Dopingconcentration was controlled by C-V measurements at the standardfrequency of 1 MHz (HP 4192A LF).

Electrical characterization versus temperature was performed by using anOxford cryostat. The measurements were performed on selected deviceswhich passed reliability tests working 1000 hours in stress conditions.

Results

In order to avoid any problems related to fabrication process andsurface preparation, devices were selected with behavior close to ideal.

A first electrical characterization was done by standard I-V and C-Vmeasurements fitting data according to thermoionic emission theory (TET)in order to obtain ideality factor, barrier height, doping concentrationand reverse current at 600 V. The mean values were 1.02, 1.21 eV, 810¹⁵cm⁻³ and 30 μA. Break down voltage was higher than 1100 V, close to theideal value. Twenty selected devices were tested further by voltagemeasurement versus time up to 1000 hour. The results demonstrated anoptimal stability with unchanged characteristic.

Electrical measurements were performed in the range 77-300 K with atemperature step of 20 K by means of forward and reverse current voltageanalysis. FIG. 2 reports the semi logarithmic plot of the I-V curves ofSBD, showing a linear behavior over seven orders of magnitude.

Table 1 reports barrier height and ideality factor versus temperatureextracted from experimental data, according to TET and by using theclassical value of 146 A K⁻² cm^(−2.) It is worth noting that theideality factor is close to 1.1 up to 77 K, and in the range 200-300 Kis almost constant. This leads to the conclusion that the barrier/metalinterface is very homogeneous which is confirmed also by the Schottkyheight barrier value obtained by C-V measurements (1.2 eV).

TABLE 1 EXPERIMENTAL VALUES OF SCHOTTKY BARRIER HEIGHT AND IDEALITYFACTOR. TEMP BARRIER HEIGHT (K) (eV) IDEALITY FACTOR 77 1.11 ± 0.2 1.13± 0.2 100 1.14 ± 0.2  1.1 ± 0.2 120 1.15 ± 0.2 1.09 ± 0.2 140 1.17 ± 0.11.07 ± 0.1 160 1.17 ± 0.1 1.07 ± 0.1 180 1.19 ± 0.1 1.05 ± 0.1 200 1.20± 0.1 1.04 ± 0.1 220 1.20 ± 0.1 1.04 ± 0.1 240 1.21 ± 0.1 1.03 ± 0.1 2601.21 ± 0.1 1.03 ± 0.1 280 1.21 ± 0.1 1.03 ± 0.1 300 1.21 ± 0.1 1.03 ±0.1

In order to obtain additional information, for the determination of theeffective barrier height and of the AA product, saturation currentdensity was extracted from experimental data in the range 200-300 K andreported in a Richardson's plot (FIG. 3).

From the slope of the linear fit and from the intercept, one can obtainan effective value of barrier height of 1.16±0.1 eV and an effectiveRichardson's constant A=17±8 A K⁻² cm^(−2.) The Richardson's constant isdifferent from the classical theoretical value. Many have tried toexplain and model the Schottky contacts on SiC, in order to fitexperimental data to the theoretical value. In theory, the value of theeffective Richardson constant A* should be calculated for asemiconductor with indirect energy gap and a number M_(C) of equivalentellipsoidal constant energy surface in the first Brillouin zone, byconsidering diagonal components of the effective mass tensor. Such atheoretical calculation leads to Richardson constant values that aredependent on the particular crystallographic direction considered inThermionic emission. The commonly reported value for the 4H—SiCRichardson constant was firstly calculated by loth et al., ISPSD '95(1995) 101-106. It can be shown that such a value was obtained by simplyaveraging the effective mass value in Gotz et al., J. Appl. Phys., 73,No. 7, (1993) 3332-2228. neglecting any crystallographic anisotropy, andconsidering a number of equivalent conduction band minima equal to 6(derived from the conduction band minimum location along the M-K edge ofthe first Brillouin zone reported in Tairov et al.,“Electroluminescence”, J. I. Parkov, Ed., Berlin-Heidelberg-New York,Springer-Verlag (1977). The location of the equivalent conduction bandminima has been more recently demonstrated to be exactly at the M-pointof the first Brillouin zone, and symmetry considerations lead to anumber of equivalent minima M_(C) equal to 3 (the constant energysurfaces are 6 semi-ellipsoids). Such a value for M_(C) should beemployed in the calculation of the effective Richardson constant of4H—SiC.

Moreover, a modified Richardson constant A** should be derived from A*taking into account quantum mechanical tunneling and reflections at theSchottky interface and interactions of the emitted electrons withoptical phonons, as suggested in Crowell et al., Solid-State Electron.,9 (1966) 1035-1048.

Such a complete model has never been applied to hexagonal materials, andto 4H—SiC in particular. Thus, the commonly accepted value of 146A/cm²V² was employed because it was estimated that an error of 30% in A*would affect the value of Schottky barrier height φ_(B) extracted fromthermionic emission saturation current measurements of about 1%.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention be limited not by thespecific disclosure herein, but only by the appended claims.

What is claimed is:
 1. A semiconductor device, comprising: a SiCsubstrate of one conductivity; a silicon face SiC epitaxial body of saidone conductivity formed on a first surface of a said SiC substrate; aSchottky metal barrier formed on said silicon face of said SiC epitaxialbody; a back power electrode on a second surface of said SiC substrateopposite said first surface of said SiC substrate.